Soluble and solid organic matter effects on atrazine adsorption in cultivated soils

Soil organic matter can be divided into solid (SOM) and water-dissolved (DOM) fractions; both of which can associate with herbicides. The objective was to examine the effects of DOM in applied solutions on atrazine adsorption in soil containing various SOM contents. A sandy loam with low (0.14%, Soil L), medium (1.2%, Soil M), or high (6.7%, Soil H) total organic matter, and artificial soil (Soil A) with no organic matter were used. These soils were treated with several different atrazine concentrations, dried, and extracted with water; the DOM concentrations in these extracts were 0, 10.1, 41.1, and 156.5 mg L-1, respectively. These extracts and the untreated soils were used in batch experiment. The distribution coefficients (K-d) for atrazine of Soils L, M, and H under mixing with Soil A extract were 0.19, 0.52, 2.51, respectively, indicating that the higher the SOM content, the higher the atrazine affinity to the soil solid phase. The effects of the DOM concentration on atrazine adsorption were estimated from the ratios (R-d) between the K-d for applied solution with no DOM and those for applied solutions containing DOM. R-d > 1 indicates that the DOM decreases the herbicide adsorption. The R-d values for Soils L, M, and H, each soil mixed with its extract, were 2.68, 1.93, and 1.07, respectively. In contrast, in the case where the soil was mixed with a solution that contained a higher DOM concentration than the soil extract, an increase in the DOM concentration in the applied solution decreased the R-d value. This was probably because of adsorption of DOM-atrazine complexes on the SOM.